Eva Enkelmann

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Papers by Eva Enkelmann

Mid-Pleistocene climate transition drives net mass loss from rapidly uplifting St. Elias Mountains, Alaska

Proceedings of the National Academy of Sciences of the United States of America, Jan 23, 2015

Erosion, sediment production, and routing on a tectonically active continental margin reflect bot... more Erosion, sediment production, and routing on a tectonically active continental margin reflect both tectonic and climatic processes; partitioning the relative importance of these processes remains controversial. Gulf of Alaska contains a preserved sedimentary record of the Yakutat Terrane collision with North America. Because tectonic convergence in the coastal St. Elias orogen has been roughly constant for 6 My, variations in its eroded sediments preserved in the offshore Surveyor Fan constrain a budget of tectonic material influx, erosion, and sediment output. Seismically imaged sediment volumes calibrated with chronologies derived from Integrated Ocean Drilling Program boreholes show that erosion accelerated in response to Northern Hemisphere glacial intensification (∼2.7 Ma) and that the 900-km-long Surveyor Channel inception appears to correlate with this event. However, tectonic influx exceeded integrated sediment efflux over the interval 2.8-1.2 Ma. Volumetric erosion accelera...

Late Miocene accelerated exhumation of the Daliang Mountains, southeastern margin of the Tibetan Plateau

International Journal of Earth Sciences, 2014

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Constraining the area of rapid and deep-seated exhumation at the St. Elias syntaxis, Southeast Alaska, with detrital zircon fission-track analysis

Tectonics, 2014

ABSTRACT [1] The Chugach-St. Elias orogen, southeast Alaska/southwest Yukon, formed in response t... more ABSTRACT [1] The Chugach-St. Elias orogen, southeast Alaska/southwest Yukon, formed in response to the ongoing Yakutat microplate subduction-collision with the North American Plate. Due to heavy glaciation, the region is a prime location to study active convergent orogenesis and climate-tectonic interactions. This study focuses on the long-term distribution of deformation in the St. Elias syntaxis area, where dextral motion along the plate bounding Fairweather Fault transitions into convergence. We present 2718 new zircon fission-track single grain ages from 26 glacio-fluvial outwash samples. The grain ages range from 293 Ma to 0.2 Ma and each sample contains three to five age populations with peaks between 267 ± 64 Ma and 1.2 ± 0.7 Ma (1σ). The rocks of the Yakutat microplate are dominated by latest Cretaceous and Eocene fission-track ages, while the rocks of the North American Plate in the syntaxis region reveal two exhumation phases at ~5.1 Ma and ~2.7 Ma. The spatial pattern of ≤5 Ma cooling ages shows that the area of rapid and deep exhumation at the St. Elias syntaxis is more extensive than previously known and confined to the south by the Fairweather Fault, to the west by the Seward Glacier catchment, and to the east possibly by the inferred Connector Fault. The area seems to be ~4800 km2 large and may extend farther to the north. The new zircon fission-track data further suggest a transpressional plate boundary since ~30 Ma and the onset of plate collision 15–12 Ma.

How was the Triassic Songpan-Ganzi basin filled? A provenance study

by Eva Enkelmann and A. Weislogel

Tectonics, 2007

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Low-temperature thermochronology of the Yakutat plate corner, St. Elias Range (Alaska): bridging short-term and long-term deformation

Quaternary Science Reviews, 2015

ABSTRACT This study investigates the long-term deformation and rock exhumation of the subducting ... more ABSTRACT This study investigates the long-term deformation and rock exhumation of the subducting Yakutat microplate corner at the St. Elias syntaxis region with a special focus on the area east and west of Yakutat Bay, where major coseismic surface uplift occurred during a MW 8.2 earthquake in 1899. We use low-temperature thermochronology to quantify the long-term activity of thrust and reverse faults. We present new apatite and zircon (U–Th–Sm)/He ages from 35 bedrock samples collected along profiles crossing major faults that compose a one-sided flower structure at the northern end of the Fairweather Fault. The cooling ages reveal that rapid rock exhumation is accommodated by the eastern Esker Creek Fault and the Yakutat Fault with rates of 2–3 mm/yr and slower rates of &lt;1.5 mm/yr occur along the Boundary Fault. Exceptionally high exhumation rates of 3–5 mm/yr are recorded by &lt;1 Ma apatite ages at the western part of the Esker Creek Fault and the Chaix Hills Fault. This area is closest to the intersection line of the northwest striking reverse faults associated with the northern Fairweather Fault and the shallow dipping northeast striking thrusts of the Pamplona-Malaspina fault zone. The long-term exhumation patterns derived from this new thermochronology data are compared with the observed coseismic uplift pattern caused by the 1899 earthquake, revealing good agreement and supporting the speculation that the 1899 rupture propagated along the western part of the Esker Creek Fault and the Chaix Hills Fault. Thus, this study illustrates that low-temperature thermochronology can support seismic hazard analyses particularly in area where preservation of paleoseismic field evidence is challenged.

Active Tectonics of An Onshore-Offshore Fold-Thrust Belt Undergoing Intense Mass Redistribution by Erosion and Deposition: Initial Results from the ST. Elias Erosion and Tectonics Project (STEEP)

Nascent Development of Focused Rock Uplift and Erosion in the St. Elias Orogen, Alaska

DEPOSITION, PROVENANCE, AND EXHUMATION OF NEOGENE STRATA IN THE SYNTAXIS OF THE CHUGACH-ST. ELIAS RANGE, SOUTHEAST ALASKA

Neogene Sediment Transport, Deposition, and Exhumation from the Southern Alaska Syntaxis to the Eastern Aleutian Subduction Zone

Evaluating the role of Cenozoic flat-slab subduction processes on the regional tectonic, sedimentary basin, and climatic framework of southern Alaska

ABSTRACT The recent recognition of flat-slab subduction along the southern margin of Alaska and n... more ABSTRACT The recent recognition of flat-slab subduction along the southern margin of Alaska and northwestern Canada has prompted a reevaluation of the Cenozoic tectonic development of this plate boundary. In this talk, we explore the upper-plate record of flat-slab subduction in southern Alaska by integrating stratigraphic, provenance, geochronologic, and thermochronologic data from the region directly above and around the perimeter of ongoing flat-slab subduction. These datasets document a change from regional Paleocene-Oligocene subduction-related magmatism and basin development to an absence of magmatism and initiation of rock exhumation that continues today. We infer that flat-slab subduction is the driving force for much of the transpressive crustal shortening, exhumation, inversion of sedimentary basins, and cessation of magmatism above the area of ongoing flat-slab subduction. Surface uplift and erosion within the flat-slab region resulted in deposition of thick, clastic wedges in sedimentary basins located along the western and northern perimeters of the flat-slab region. Along the eastern perimeter, northwestward-propagating Oligocene-Quaternary slab-edge volcanism and transtensional basin development along dextral strike-slip faults record progressive insertion of a shallow slab. The southern perimeter of the flat-slab region is characterized by active deformation, focused glacial erosion, deposition of thick packages of Neogene glacial marine sediment, and rapid exhumation along thrust belts and accretionary prisms. Collectively, all these geologic data indicate that Cenozoic flat-slab subduction had a direct role in regional deformation and basin development along the southern Alaska convergent margin. An interesting but still poorly understood question is the possible role of flat-slab subduction processes on the regional Cenozoic climate of southern Alaska. From our perspective, the presence of over 5 km of late Miocene to Pliocene glacial strata overlying an angular unconformity along the southern perimeter of the flat-slab region marks the initiation of tectonically created topography high enough to generate glacial conditions. Along the northern perimeter basin of the flat-slab region there is a documented change in forest composition from a warm, temperate, deciduous-dominated forest to a cooler, conifer-dominated forest that coincides with the middle-late Miocene uplift of the central Alaska Range. In general, during early and middle Miocene time, perimeter basins were dominated by lacustrine/paludal/fluvial environments suggesting little spatial partitioning of moisture across the flat-slab region. During late Miocene-Pliocene time, alpine glaciation developed in the Alaska and St. Elias Ranges above the flat-slab region, especially on the south sides of the ranges, suggesting a link between tectonically generated topography and local climate.

Quantifying the spatial distribution of rapid exhumation from glacial detritus, St. Elias Range (Alaska)

ABSTRACT We present a provenance analysis, zircon (U-Th)/He thermochronometer cooling ages, and p... more ABSTRACT We present a provenance analysis, zircon (U-Th)/He thermochronometer cooling ages, and petrologic characterization of exhumation processes active under the Malaspina Glacier, St. Elias Range, Alaska. The collision of the Yakutat Terrane with North America formed the St. Elias Range in southeast Alaska. This region is subject to active faulting, high seismicity, and intense glacial erosion. Recent thermochronology studies document rapid rock exhumation at the Yakutat corner where strike-slip deformation transitions to convergence. Published detrital zircon fission track ages from sand samples of the Seward - Malaspina Glacier yielded a young age population of 2 Ma, suggesting a region of rapid exhumation somewhere in the glacier drainage basin. However, the glaciers cover the area and prevent direct observation and sampling, thereby inhibiting detailed bedrock exhumation studies of the Yakutat corner region. We investigate the lithologies of the bedrock at the Yakutat corner to quantify patterns of rock exhumation using clasts collected from the toe of the Malaspina Glacier. Seven different locations 50 km around the glacier were sampled and lithologies characterized by point counting in the field. In total, 1998 clasts were analyzed. We identified six main lithological groups that are assignable to specific geological units. Most of the detritus is gneiss and granulite (26%), micashist and phyllite (26%), and granite (11%) and are typical lithologies of the Chugach Terrane located at the southern margin of the North American plate. Mafic rocks of amphibolite, gabbro, and basalts (21%), and metapelite, sandstone, quartzite and breccia (14%) were also present and are typical for the colliding Yakutat Terrane. Two percent of the clasts are tightly foliated and mylonitic. These clasts are inferred to originate from the Contact Fault, an old suture zone that has been reactivated during Yakutat Terrane collision. Individual clasts from each sample locality and from all lithological groups were measured for zircon (U-Th)/He thermochronometer cooling ages. Furthermore, thin sections of 80 clasts were analyzed for petrographic characterization of the lithological groups. The zircon and titanite grains of 50 clasts have been separated and prepared for U-Th/He dating, degassed for He, and await final measurement of U-Th concentrations. We will report a comparison of these cooling ages with published bedrock and detrital low-temperature thermochronometer ages from neighboring areas. These cooling ages and their associated lithologies will allow identification of the spatial distribution of exhumation and the different magnitudes of exhumation between the two plates (Chugach vs. Yakutat).

Exhumation of the northwestern Fairweather Range, southeast Alaska

ABSTRACT The study investigates spatial and temporal patterns in exhumation along the transpressi... more ABSTRACT The study investigates spatial and temporal patterns in exhumation along the transpressive plate boundary Fairweather Fault, Alaska, USA. The Fairweather transform fault forms the eastern boundary between the obliquely colliding Yakutat Terrane and the North American plate. Deformation on the Fairweather Fault initiated around 30 Ma and has been suggested to be a transpressive dextral strike-slip fault zone that formed the Fairweather Range. This 300 km long and 50 km wide mountain range is characterized by &gt; 4000 m high peaks and deep glacial valleys that drain into the North Pacific. Previously published low-temperature thermochronometer data suggest that deformation and exhumation is not restricted to one side of the fault. Maximum estimated exhumation rates of up to 2 mm/yr occur within 10 km of the fault and extend about 300 km along strike. We collected 15 sand samples from modern glacial drainages spanning 80 km along strike of the northwestern part of the Fairweather Range to quantify spatial variations in deformation and exhumation of this range. Zircon fission track age distributions are measured to quantify the long-term exhumation patterns along this fault. The aim of this study is to gain better special coverage for the cooling age signals in this glaciated and remote region. We concentrate on the northern part of the Fairweather Range because it comprises the partitioning of strain as the strike-slip motion of the Fairweather Fault changes to convergence and thrust tectonics when it merges into the St. Elias Range. Calculation of final grain-age distributions from these samples is in progress. We will present detrital zircon fission track ages along the northwestern ca. 80 km of the Fairweather Fault and including the transition into the St. Elias Range. Based on existing bedrock samples we anticipate the youngest age populations to be &lt;5 Ma and in catchments that cover the Fairweather Fault itself. Older age populations are also anticipated &gt;15 Ma in catchments that are located farther away from the fault. Additionally, we expect to see changes in the amount of exhumation and its distribution with respect to the fault zone along the strike. Deviations in the new detrital samples compared to existing bedrock samples could be due a) localized areas of enhanced or retarded exhumation due to temporal variations on Fairweather Fault slip, b) deformation and exhumation of rocks on previously unrecognized structures, and/or c) spatial variations in the intensity of glacial erosion along strike of the Fairweather Fault.

Luminescence thermochronometry and (U-Th-Sm)/He dating across the Fairweather Fault (Russell Fjord, Alaska)

Crustal flow around the Eastern Himalayan Syntaxis in western Yunnan, China

by Eva Enkelmann and B. Sonntag

The exhumation of the westernmost Tien Shan (Tajikistan, Uzbekistan)

U-Th/He dating of the Cenozoic Uplift of Sinai

ABSTRACT In relationship to the opening of the Red Sea, Sinai Peninsula (Egypt) experienced subst... more ABSTRACT In relationship to the opening of the Red Sea, Sinai Peninsula (Egypt) experienced substantial uplift and erosion in the younger Cenozoic producing a topographic relief of over 2500 m in the southern part of the Peninsula. In order to document how much uplift and erosion occurred in total, a vertical profile covering about 2500 m in elevation was dated. U-Th/He dating was used because of its very low closure temperature of 60°C. We found that uplift was enough to penetrate the 60°C isotherm, at least in the lowest elevation parts of the region of highest topographic relief around Mount Catherine in southern Sinai. We already presented these results at EGU2012. In order to expand upon these results and understand the horizontal distribution of surface uplift across Sinai, we dated 19 further samples along five profiles on the west and east coast, with the purpose to find the level of the prior 60°C isotherm. 200 vertical metre intervals were used for the sampling. Analytical results are currently being processed and will be presented at EGU. The idea after which we took the samples was to get at least one sample per profile above, and at least one sample below the former 60°C isotherm prior to the Cenozoic denudation history. It is planned that these results will allow to construct a two dimensional distribution of uplift across Sinai. The final goal is, in conjunction with published fission track ages, to get a pretty detailed image of the uplift of Sinai for the Cenozoic.

Spatial distribution of exhumation at the Yakutat plate corner and the role of glacial erosion, southeast Alaska/southwest Yukon

Deformation record of 4-d accommodation of strain in the transition from transform to oblique convergent plate margin, southern Alaska

ON THE PROVENANCE OF SEWARD-MALASPINA GLACIER COBBLES AND THE EXTENT OF RAPID, DEEP EXHUMATION AT THE YAKUTAT PLATE CORNER, SE ALASKA

Introduction: Neogene tectonics and climate-tectonic interactions in the southern Alaskan orogen themed issue

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